Radiofrequency (“RF”) transmit coils used in magnetic resonance imaging (“MRI”) are normally designed to be as efficient as possible, typically characterized by the “Q” of the coil. In particular, RF transmit coils are designed to be efficient at converting power that is provided to the coil into the transmitted RF power. In addition, RF transmit coils need to be tuned so that their peak efficiency occurs close to the expected resonance frequency of the resonant species that is to be investigated. For example, when imaging protons (i.e., the most commonly imaged resonant species), the RF transmit coil needs to be tuned to approximately 42.57 MHz per Tesla of the magnetic field strength of the MRI system's main magnetic field, B0. However, the width of this resonant tuning also tends to scale with magnetic field strength, so that the RF transmit coil is efficient over a wider range of frequencies at higher magnetic field strengths, but is efficient over a smaller range of frequencies at lower magnetic field strengths.
In addition, when performing slice-selective excitations in normal MRI applications, when an off-center slice needs to be excited the transmitted RF pulse may need to be applied with an offset frequency from the nominal resonance frequency of the target resonant species. In these situations, the transmit frequency is offset based on the distance of the slice from center, the transmit bandwidth of the RF pulse (which is most commonly influenced by the duration of the RF pulse), and the strength of the applied slice-selective gradient. The net result is that for thin, off-center slices excited with short RF pulses, the RF excitation pulse may need to be transmitted at a large offset frequency.
Some MRI systems implement an asymmetric gradient design, in which the gradient does not produce a net zero field that coincides spatially with the magnet isocenter. In these systems, the asymmetric gradient acts like a nominal slice offset in the corresponding direction, which can require a large offset frequency even for slices excited at isocenter because of the additional non-zero gradient in the asymmetric gradient direction.